1. Field of the Invention
The invention concerns a semi-automatic method of extracting a contour of a contrasted area in a digital image stored in computer memory, in particular a digitized photographic image.
The invention applies in particular to iterative systems aiding the extraction and identification of networks of roads or rivers or coastlines in a digital satellite or aerial image.
2. Description of the Prior Art
The article "Using Dynamic Programming for Minimizing the Energy of Active Contours in the Presence of Hard Constraints" by Amir A. Amini et al, IEEE, Second Int. Conference on Computer Vision, 5 Dec., 1988, TAMPA (Fla.), pages 95-99, describes a contour extraction method based on an iterative process of deforming an active contour using an energy minimizing function. The article "Towards Dynamic Adaptation of Snakes Contours" by Marie-Odile Berger, International Conference Image Analysis & Processing", COMO (Italy), September 1991 also refers to a semi-automatic contour extraction method of this kind.
This prior art method includes the input of an initial contour into the digital image near the contour to be extracted. This initial contour is in the form of a polygonal trace that seeds the iterative process. The iterative process deforms the initial contour until it finds an equilibrium position corresponding to an energy minimum, this equilibrium position defining a part of the trace of the contour to be extracted. This part of the trace is then extended by a straight line segment, the combination being deformed again and this process repeated on each iteration until the process is halted by the operator. On each iteration of the process a function extends the current trace in an extension direction determined by local analysis of the digital image at the end point of the current trace. This local analysis of the image at the current point is conventionally based on determining the tangent to the trace at the current end point or determining the perpendicular to the gradient at the current end point.
The manner in which the current trace is extended is not always satisfactory. In particular, if the contour to be extracted has sudden variations in curvature, the trace tends not to follow these curvature variations. It is then necessary to intervene manually in the deformation process or the active contour extension process to assist the trace to rejoin the contour to be extracted. Such intervention is time-consuming if it is required frequently. This is the case when the contour to be extracted in a digital image is particularly complex, for example when it is a coastline.